In a typical landfill a portion of the biodegradable material decomposes and eventually is transformed into solid residuals, landfill gas, and/or leachate. Aerobic bacteria initially decompose the waste until the available oxygen is consumed. This stage usually lasts for a short time and is followed by the anaerobic acid state, in which carboxylic acids accumulate, the pH decreases, and some cellulose and hemicellulose decomposition occurs. Finally, during the methanogenic state, bacteria further decompose the biodegradable material into methane and carbon dioxide. These are the typical biological processes by which the waste mass of a landfill decomposes over time. Each of these biological processes generates heat. If a landfill overheats a self-sustaining exothermic reaction may be initiated.
In addition to the biological processes described above, other processes generate heat in the landfill, such reactions include 1) aerobic consumption of waste wherein the cause is often associated with an operational issue such as disposal of hot material and/or the over application of vacuum on a gas extraction well (a typical landfill subsurface “fire”), 2) exothermic chemical reactions such as when water is combined with certain wastes, such as aluminum production wastes, lime, steel mill waste, and other metal wastes, and, 3) anaerobic, pyrolytic reactions which cause thermal decomposition of the waste and may release heat under certain conditions.
Reactions such as those described immediately above are problematic in that they generally produce adverse impacts. These adverse impacts can include odors, smoke, fugitive emissions, liner or cap damage, gas and leachate management structural damage, excessive settlement, slope failure, ground water and/or surface water impacts; and disruption of landfill operations. In addition, subsurface reactions tend to increase leachate generation, sometimes by an order of magnitude, which may result in outbreaks, where excessive leachate exits the waste mass at locations that can create an environmental hazard. As such, controlling the spread of these subsurface reactions is critical to reducing potential landfill air emissions as well as potential adverse impacts to ground and surface waters.
Typical landfill fires (described as stemming from an operational issue above) are extinguished using well-accepted industry standard operating practices which eliminate the oxygen that is needed to sustain the fire. However, the reactions described above as exothermic chemical reactions and pyrolytic reactions occur in the absence of oxygen (anaerobic), so the standard practices for fires do not apply.
There are no easy short term fixes to landfill subsurface pyrolytic reactions and the attendant adverse impacts of those reactions. For example, excavation of the hot waste can result in the threat of fire from the introduction of oxygen and in most cases excavation of the hot waste may not be a feasible option if the subsurface reaction is very deep, extensive or rapidly propagating. Opening a landfill to address the reaction also results in exposing the waste mass to vectors such as birds and rodents that can create a broad set of undesirable ecological impacts. Measures taken to decrease temperatures have been shown to work more rapidly to suppress the subsurface reaction than measures taken to exclude oxygen. Further discussion on this control strategy will be detailed in the Summary section below.
Subsurface reactions can be self-sustaining high-temperature reactions that consume waste underground, producing rapid “settlement” of the landfill's surface. Deep-seated subsurface reactions do not “burn,” instead these events are believed to be a form of thermal decomposition known as pyrolysis, under which the thermal reaction takes place in an oxygen starved environment and the reacting material is consumed slowly and at relatively low temperatures. Subsurface reactions are generally defined as the sustained pyrolysis of carbon based material at elevated temperatures accompanied by the evolution of heated gaseous products.
A significant impact of a subsurface reaction is that substantial settlement of the waste mass can occur over a short period of time. This settlement occurs due to the reduction in the volume of the waste mass from pyrolysis of the waste mass resulting in greater than normal settlement over and adjacent to the reacting waste mass. In addition, substantial settlement can occur due to the generation of and dissipation of pressure within the waste mass resulting from the phase change of liquid entrained in the waste mass to vapor phase.
A subsurface reaction can also result in direct impact on engineered components, thermal damage to the engineered components and as discussed above, differential settlement of the engineered components. If the reduction in the waste volume due to the pyrolysis is significant, it can lead to the settlement of the overlying waste materials. Consolidation and settlement of the waste materials can lead to subsidence and differential settlement of the engineered landfill cover. Differential settlement of the engineered landfill cover can result in damage to the cover system which can negatively affect the performance of the landfill cover through desiccation, creation of cracks, or in the extreme complete disruption resulting in offsets in the cover system layers.
Preventing the occurrence, or limiting the advancement of the subsurface reaction is a mechanism for reducing the environmental impacts of the reaction and to reduce the adverse impacts of differential settlement. To limit the movement of the subsurface reaction, heat must be removed from the waste mass thereby retarding or stopping the advance of the heat front, the leading edge of the subsurface reaction as it expands, by stalling or eliminating the exposure to new waste mass to higher temperatures. It is well known in the industry that to accomplish heat removal from a landfill that a piping loop can be installed within the waste mass that circulates temperature treated water or water based solutions. Exemplary of this technique are the disclosures of U.S. Pat. Nos. 8,672,586 and 9,255,727 by Yesiller. In the Yesiller references, the circulation fluid extracts heat as the fluid circulates through the closed-loop, returning the warmer circulation fluid to a heat exchanger for cooling and recirculation. The system and methods disclosed therein are inadequate to address the significant and aggressive differential settlement that occurs within landfills afflicted with a subsurface reaction.